The present invention relates to systems for monitoring spaces by electro-optical means.
Electro-optical correlators useful in the present invention are disclosed in U.S. Pat. Nos. 3,781,110; 3,856,400 and 3,856,401, the disclosures of which are incorporated herein.
Various systems are already known for monitoring spaces. There are electrostatic, electromagnetic, mechanical, electro-mechanical and optical methods operating on the principle of light barriers. One of the drawbacks of the electrical methods consists in their susceptibility to interference by strong electrical errors from external sources. Electrostatic equipment in enclosed rooms is sensitive to changes in the rooms and must be readjusted each time. This electrostatic equipment is also affected by atmospheric humidity, particularly as regards open areas. Methods to date for traffic monitoring make use of copper loops inserted into the traffic path, the induction fields of which are affected by metal, such as iron, in the vehicles. These devices suffer from the drawback that only restricted spaces may be monitored and their installation requires tearing up the surface of the traffic path. Humans and animals on sidewalks or on the traffic path are not detected. Furthermore, vehicles are detected only when they contain metal. This may be a source of difficulty in the future when there is wider use of plastics. Electro-mechanical switches as used underneath mats for the automatic closing of doors suffer from the drawback of a break in the floor, difficult and costly installation, and unfavorable cleaning. Since a system, not part as a unit of automatic door and elevator control, is used, it is complex in installation. Light barriers only monitor restricted spaces which at best are two-dimensional and comprise gaps and which are sensitive to drift. German Pat. No. 663,931 discloses an electro-optical sensor with a spatial frequency filter, which detects motion. This instrument suffers from the drawback that it may be spuriously affected by lights and shadows projected into the space being monitored. Furthermore, it fails to detect motion toward the optics. It also fails to record whether there is occupancy of a space being monitored unless there be detectable measurement of a body penetrating this monitored space.
German Pat. No. 663,931 further discloses the feasibility of ascertaining the direction of motion by means of a spatial frequency filter the scanning frequency of which is varied over the image field. This, however, entails three drawbacks:
a. ascertaining the direction of motion is feasible only if the object covers only part of the image field so that the increase or decrease in frequency unambiguously discloses the direction of motion;
b. ascertaining the direction of motion is only satisfactorily feasible if a constant motion of the object is expected, since for instance a very quickly accelerating object produces the spurious effect of the opposite direction; and
c. a varying scanning frequency is obtained only by trading against the signal constancy and hence signal quality (see L. M. Biberman, "Reticles in Electro-Optical Devices," Pergamon Press, London 1966).